Nandor (singular Nando) were Elves of Telerin descent, who left the Great Journey from Cuiviénen to Valinor when the Elves reached the Misty Mountains. Under their leader Lenwë (Dan in their own language), a group of Teleri turned south along the Great River Anduin, and disappeared from written history. Nandor eventually became their term for themselves, and meant "People of Dan" in their own language.
Many years later a group of Nandor under Denethor, son of Lenwë, crossed the Blue Mountains into Ossiriand, which was then called named Lindon, or "Land of the Singers", after these Elves. They became known as the Laiquendi (singular Laiquendë) or Green Elves.
The Silvan Elves of Mirkwood and Lothlórien were descended from the Nandor (but most of their lords were not), as were the Elves which dwelt at Edhellond near Dol Amroth during the early days of Gondor.
Nandorin, the language of the Nandor, gradually disappeared from Middle-earth after the end of the First Age, when Sindarin elves merged with the Silvan folk and were taken as their lords. Nandorin/Silvan gradually became extinct, surviving only in placenames such as Laurelindórinan and Lindórinand (old names for Lórien) and proper names such as Amroth. The daily tongue of the Silvan elves became Sindarin, or Sindarin with some Silvan influences.